Abstract

Background

Although molecular analyses have contributed to a better resolution of the animal
tree of life, the phylogenetic position of tardigrades (water bears) is still controversial,
as they have been united alternatively with nematodes, arthropods, onychophorans (velvet
worms), or onychophorans plus arthropods. Depending on the hypothesis favoured, segmental
ganglia in tardigrades and arthropods might either have evolved independently, or
they might well be homologous, suggesting that they were either lost in onychophorans
or are a synapomorphy of tardigrades and arthropods. To evaluate these alternatives,
we analysed the organisation of the nervous system in three tardigrade species using
antisera directed against tyrosinated and acetylated tubulin, the amine transmitter
serotonin, and the invertebrate neuropeptides FMRFamide, allatostatin and perisulfakinin.
In addition, we performed retrograde staining of nerves in the onychophoran Euperipatoides rowelli in order to compare the serial locations of motor neurons within the nervous system
relative to the appendages they serve in arthropods, tardigrades and onychophorans.

Results

Contrary to a previous report from a Macrobiotus species, our immunocytochemical and electron microscopic data revealed contralateral
fibres and bundles of neurites in each trunk ganglion of three tardigrade species,
including Macrobiotus cf. harmsworthi, Paramacrobiotus richtersi and Hypsibius dujardini. Moreover, we identified additional, extra-ganglionic commissures in the interpedal
regions bridging the paired longitudinal connectives. Within the ganglia we found
serially repeated sets of serotonin- and RFamid-like immunoreactive neurons. Furthermore,
our data show that the trunk ganglia of tardigrades, which include the somata of motor
neurons, are shifted anteriorly with respect to each corresponding leg pair, whereas
no such shift is evident in the arrangement of motor neurons in the onychophoran nerve
cords.

Conclusions

Taken together, these data reveal three major correspondences between the segmental
ganglia of tardigrades and arthropods, including (i) contralateral projections and
commissures in each ganglion, (ii) segmentally repeated sets of immunoreactive neurons,
and (iii) an anteriorly shifted (parasegmental) position of ganglia. These correspondences
support the homology of segmental ganglia in tardigrades and arthropods, suggesting
that these structures were either lost in Onychophora or, alternatively, evolved in
the tardigrade/arthropod lineage.